1. Field of the Invention
The invention relates to a five-level DC-AC converter and, in particular, to a five-level DC-AC converter with a simpler circuit configuration and a lower component cost.
2. Description of Related Art
Energy development and applications are the important topics in the world. Many countries make efforts to find new energy sources or renewable energy in view of depletion of fossil energy someday. In line with environmental protection issues, the renewable energy includes solar power, wind power, etc. Such energy sources use the natural energy and convert it into electric power. Within the devices that convert the natural energy into electric power, DC-AC converter is an indispensible component.
The general power system is based on AC power. The DC-AC converter can conveniently convert the DC power output by various generators into AC power and feed the AC power into the power system for supplying power. Conventionally, the DC-AC converter is a two-level or a three-level converter implemented by half-bridge power converter or full-bridge power converter. Recently, multi-level DC-AC converters have been developed.
With reference to FIG. 1, a single-phase DC-AC converter is implemented using a single-phase three-level diode-clamped power converter. The single-phase three-level diode-clamped power converter includes two DC capacitors 50 and 51, two half-bridge circuits 53 and 54, two diodes 55 and 56.
Two DC capacitors 50 and 51 are connected in series, and the two ends of the two DC capacitors 50 and 51 are connected to a DC bus 52, providing V+ and V− terminals connecting to an external DC power source.
The two half-bridge circuits 53 and 54, comprise two power electronic switches connected in series, are connected in series and then further connected to the DC bus 52. Serial connection nodes of two power electronic switches in half-bridge circuits 53 and 54 are connected to the serial connection node of the two DC capacitors 50 and 51 through diodes 55 and 56, respectively. A serial connection node of the two half-bridge circuits 53 and 54 is connected to an inductor. The inductor and the serial connection node of the two DC capacitors 50 and 51 are connected to an AC output port 57.
The single-phase three-level diode-clamped DC-AC power converter converts the DC voltage on the two DC capacitors 50 and 51 into AC voltage via turning on or off the power electronic switches of two half-bridge circuits 53 and 54. The voltage variation caused by each switching operating of the power electronic switches is equal to the voltage of one DC capacitor 50 or 51. The output AC voltage thus has variations of three levels. When the load is light, the voltages of the two DC capacitors 50 and 51 are not easy to be balanced. Therefore, the control of single-phase three-level diode-clamped power converter is complicated when the voltage balance of the two DC capacitors 50 and 51 is considered to obtain a stable AC output.
With reference to FIG. 2, a single-phase three-level flying-capacitor power converter is used as a single-phase three-level DC-AC converter. It is mainly composed of two DC capacitors 60 and 61 and two half-bridge switch circuits 62 and 63. The two DC capacitors 60 and 61 are connected in series. The two half-bridge circuits 62 and 63, comprise two power electronic switches connected in series, are also connected in series. However, the serial connection nodes of the two power electronic switches in the two half-bridge circuits 62 and 63 are connected with another DC capacitor 64. A serial connection node of the two half-bridge circuits 62 and 63 is connected to an inductor. The inductor and the serial connection node of the two DC capacitors 60 and 61 are connected to an AC output port 65. Its circuit operation is basically the same as the above-mentioned diode-clamped power converter. However, when controlling the two half-bridge circuits 62 and 63, it is necessary to take into account the charging and discharging of the DC capacitors 60, 61 and 64 in order to regulate their voltages. Thus, the control of the single-phase three-level flying-capacitor power converter is complicated.
Apparently, the circuit configuration and the switching control for the power electronic switches of a higher-level DC-AC converter will be more complicated. With reference to FIG. 3, a five-level DC-AC converter includes two DC capacitors 70 and 71 and two full-bridge circuits 72 and 73. The two DC capacitors 70 and 71 are connected to two DC buses 701 and 702. The DC bus 701 provides V1+ and V1− terminals, and the DC bus 702 provides V2+ and V2− terminals. Accordingly, two DC power sources are required. The two full-bridge circuits 72 and 73 are connected in parallel with the corresponding DC capacitors 70 and 71. One output node of the two full-bridge circuits 72 and 73 are connected together. The other node of full-bridge circuit 72 is connected to an inductor. The inductor and the other node of full-bridge circuit 73 are connected to an AC output port 74.
According to the above-mentioned circuit structure, to output the AC power with five voltage levels, various DC capacitors 70 and 71 have to connect to V1+, V1−/V2+, V2− terminals of the DC power sources, in addition to using two full-bridge circuits 72 and 73. In short, the five-level DC-AC converter has to use two full-bridge circuits 72 and 73, including eight power electronic switches in total. In addition to the complicated circuit design and high cost, the control of the power electronic switches in the two full-bridge circuits 72 and 73 becomes more complicated under the consideration for balancing the voltages of two DC capacitors 70 and 71.